1. Field of the Invention
The present invention relates to an exposure method by which a pattern image is exposed, via a projection optical system and a liquid, on a substrate; to a substrate stage that supports a substrate; to an exposure apparatus; and to a device manufacturing method.
2. Description of Related Art
Semiconductor devices and liquid crystal display devices are manufactured through the so-called photolithography technique, by which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in the photolithography process has a mask stage that supports a mask and a substrate stage that supports a substrate, and while successively moving the mask stage and the substrate stage, transfers the mask pattern, via a projection optical system, onto the substrate. In recent years, to address the increasingly high integration of device patterns, increasingly high resolution of the projection optical system has been desired. The shorter the exposure wavelength used is, and, also, the larger the numerical aperture of the projection optical system is, the higher the resolution of the projection optical system becomes. For this reason, the exposure wavelength used for the exposure apparatus is becoming shorter and shorter year by year, and the numerical aperture of the projection optical system is also becoming larger and larger. In this context, the presently dominant exposure wavelength is 248 nm from a KrF excimer laser, but a still shorter wavelength of 193 nm from an ArF excimer laser is now gradually being put to practical use.
In addition, when performing exposure, the depth of focus (DOF) is an important factor along with the resolution. The resolution R and the depth of focus δ are respectively expressed by the following formulas:R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)where λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. It can be seen from formulas (1) and (2) that if, to enhance the resolution R, the wavelength λ is made shorter and the numerical aperture is made larger, then the depth of focus δ becomes narrower.
When the depth of focus δ becomes too narrow, it becomes difficult to make the substrate surface coincide with the image plane of the projection optical system, and thus there occurs the possibility that the focus margin during the exposure operation will be insufficient. To address this problem, the liquid immersion method, which is disclosed in, e.g., PCT International Publication No. WO 99/49504, has been proposed as a method to make the exposure wavelength shorter in effect and to make the depth of focus broader. This liquid immersion method is designed to, by filling the space between the under surface of the projection optical system and the substrate surface with a liquid, e.g., water or organic solvent, to form a liquid immersion region and thus by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times (n is the refractive index of the liquid and is generally about 1.2 to 1.6) of that in the air, improve the resolution and, at the same time, enlarge the depth of focus by approximately n times.
By the way, with the above-described related art, there are problems as described below. The above-described prior art has adopted the configuration in which the space between the image plane side end face of the projection optical system and the substrate (wafer) is locally filled with a liquid, and when exposing shot areas around the center of the substrate, the liquid does not flow out to the outside of the substrate. However, when, as shown by the schematic of FIG. 27, projection area 100 of the projection optical system is applied to peripheral area (edge area) E of substrate P for the purpose of exposing edge area E of substrate P, the liquid flows out to the outside of substrate P, which leads to the disadvantage that the liquid immersion region is not formed well, thus deteriorating the pattern image projected. Furthermore, there also arises the disadvantage that the liquid that flowed out causes mechanical parts, etc. in the vicinity of the substrate stage that supports substrate P to rust or causes electric leakage of the stage drive system, etc. Moreover, if the liquid that flowed out finds its way over to the underside surface of the substrate and penetrates into the space between the substrate and the substrate stage (substrate holder), there also arises the disadvantage that the substrate stage cannot hold the substrate well. In addition, air bubbles may come to be mixed in the liquid due to the height difference or gap between substrate P and the substrate stage, and, in this case, there arises the disadvantage, for example, that the exposure light is scattered under the influence of the air bubbles or that the pattern does not focus into an image on substrate P because of the bubbles. Besides, also when the liquid penetrates into the above-mentioned gap, rust or electric leakage may be induced.